Publications and other reference materials referred to herein are numerically referenced in the following text and respectively grouped in the appended Bibliography which immediately precedes the claims.
The increased data capacity in cloud servers requires reliable transmission at the highest available data rate. Therefore, optical communication between the server units is used due its high data rate transmission capabilities. In order to fit the aggressive low cost requirements of data centers, the next generation transceivers are based on commercially available 25G direct detection components combined with state-of-the-art communication and digital signal processing (DSP) techniques. Currently, 100G data center transceivers include a hybrid optical-electrical module packaged in a single pluggable form-factor. However, the hybrid transceiver poses system design challenges for performance optimization, package size, and power dissipation. To overcome these challenges, the next generation transceiver may include only the optical components, while the electronic blocks will be hosted externally on the transponder board. This separation enables the use of small form-factor pluggable (SFP) packages, where the aggregated optical signal consists of up to four independent channels, each carrying data at a rate of 25 Gbaud and higher. In turn, each of the optical channels is detected and converted to an electrical signal by the optical module. The electrical signal is transmitted over a printed transmission line (PTL) from the optical module to the electronic module. Both modules are hosted on the host printed circuit board (PCB), e.g., backplane.
Effective area utilization of a PCB is very important for cost-effective, compact and high port density solutions. Therefore, for effective use of the PCB area, the transmission lines (TLs) and the components should be integrated tightly. In next generation data center transceivers, each of the PTLs is expected to support 25 Gbaud and higher, i.e., ultra-broadband electrical signals. However, dense integration of the TLs in high-speed applications can intensify impairments, such as crosstalk, which in turn leads to signal degradation. The common techniques to reduce those impairments include properly separated and well-shielded TLs, or multilayer design with isolated high-speed lines. However, those techniques are tailor-made for a specific application and increase the cost and the required area of the PCB compared to low-speed applications where those impairments are negligible.
Coupled PTLs have been studied in various RF and microwave applications and accordingly compensation techniques have been proposed. Mbairi et al. studied the crosstalk between adjacent high-frequency printed transmission lines (TLs) as a function of traces separation [1]. It was shown that the crosstalk for coupled microstrips and coupled stripline varied considerably with frequency and should not be ignored. It was proposed to increase the adjacent traces separation, but this reduces the effective utilization of the PCB. On the other hand, Prachumrasee et al. studied the crosstalk between coupled microstrips in hard drive applications [2]. The crosstalk has been suppressed by using differential lines and a magnetic composite. But, the differential lines required more area compared to single-ended lines [3]. Alternatively, Pelard et al. present integrated circuit solutions that compensate for crosstalk and intersymbol interference (ISI) of high-speed data transmission over legacy systems, e.g., short reach optics and electrical backplanes [4]. They showed that crosstalk canceller (XTC) improves the bit error rote (BER) significantly and enables 10 Gbps data rates on legacy systems. Also, in order to mitigate the crosstalk impairment in coupled microstrips, Kao et al. presented a 10 Gbps parallel receiver with joint XTC and decision-feedback equalizer (DFE) [5]. It was demonstrated that the adaptive receiver can compensate for channel loss and cancel the far end crosstalk (FEXT) simultaneously. Similarly, Oh et al. designed a multiple high-speed I/Os XTC analog front-end that handles the crosstalk of coupled microstrips at 12 Gbps and improved the eye opening considerably [3]. Recently, Han et al. realized a 2×50 Gbps receiver with adaptive channel loss equalizer and XTC using a SiGe BiCMOS process [6]. The adaptive joint XTC equalizer compensated for the crosstalk and loss of the coupled microstrips with capacitive- or inductive-coupling nature. However, all the electrical analog XTC techniques in [3-6] are limited by their compensation, such that wideband signals with pronounced coupling and significant microstrip length may require a more accurate compensation model.
It is therefore a purpose of the present invention to provide inclusive methods of compensating for coupling impairments that occur in systems comprising two or more transmission lines.
It is another purpose of the present invention to provide inclusive compensation methods for the coupling impairments of coupled-pair microstrips in optical communication systems that improve the receiver sensitivity and support significantly longer microstrip traces as compared to the classical crosstalk compensation technique.
Further purposes and advantages of this invention will appear as the description proceeds.